Field of the Invention
The present invention relates to an imaging device having two photoelectric conversion units with different light-receiving efficiencies.
Description of the Related Art
A simultaneous capturing of a motion image and a static image enables a user to view a captured scene as a motion image and also enjoy a critical scene in the motion image as a static image, which can significantly enhance the value of the captured image. Further, a simultaneous capturing of a motion image of a normal framerate and a motion image of a high framerate by a single camera allows a user to enjoy a high quality production while switching a particular scene to a slow-motion video and thereby perceive full of movement.
In general, a faster shutter speed in capturing a motion image causes so called jerkiness, which is viewed like a frame-by-frame video during playback, resulting in a loss of smoothness of a video. In order to suppress such jerkiness to have a smooth video, it is necessary to set accumulation time close to one frame period in a series of capturing. That is, when the framerate is 30 fps, a relatively longer accumulation time period such as 1/30 seconds or 1/60 seconds will be appropriate. In particular, this setting is important in a situation where a position of a camera is unstable such as in an aerial capturing.
On the other hand, in a static image, it is required to capture an image having a so called stop motion effect that suppresses a blur to capture a moment. It is therefore necessary to set a short accumulation time period such as around 1/1000 second, for example. Further, since one frame period is short in a high framerate motion image, when the framerate is 120 fps, for example, a shorter accumulation time period such as 1/125 seconds or 1/250 seconds will necessarily be set.
Simultaneously capturing two images of a motion image and a static image or two images of a motion image of a normal framerate and a motion image of a high framerate through a single capturing lens means that an aperture used in these types of capturing is common to each other. Also in this case, it is desirable to obtain almost the same level of signal charges at imaging elements while two images are captured with different settings of the accumulation time period and thereby obtain both images with a good S/N ratio and less feeling of noise.
Japanese Patent Application Laid-open No. 2014-048459 discloses an imaging device including a pair of photodiodes (photoelectric conversion units) having asymmetric pupils in each pixel. In the imaging device disclosed in Japanese Patent Application Laid-open No. 2014-048459, one photodiode of the pair of photodiodes has a higher light-receiving efficiency and the other photodiode has a lower light-receiving efficiency. A use of two signals from a pair of photodiodes as separate image data enables a simultaneous capturing of two images.
In using one imaging element having two photoelectric conversion units with different light-receiving efficiencies to simultaneously capture two images, however, it is difficult to optimize exposures of two images at the same time because of the common aperture. In particular, when different shutter speeds are set for two images, optimizing the exposure of one of the two images results in overexposure or underexposure of the other.